When a semi-permeable membrane through which passes water but hardly passes dissolved materials (ion and molecule) is installed between the solution with a high concentration and the solution with a low concentration, a natural phenomenon occurs. Here, the natural phenomenon is called an “osmotic action” or “osmotic phenomenon”. M. Traube, German chemist, discovered the osmotic phenomenon in 1867, and Pfeffer measured the osmotic pressure caused by the osmotic phenomenon in 1877.
This osmotic phenomenon may be the core of the desalination technology using seawater, which is one of methods for solving worsen water shortage that is caused by a climate change due to the global warming, an increase of industrial water due to the industrialization, and an increase of water demand due to the population growth.
However, the current seawater desalination process may have a limitation in the economic aspect so far due to high energy-intensive process except for water shortage areas such the Middle East.
A method for converting seawater into fresh water by removing salts contained in the seawater may be classified into an evaporation method and a reverse osmosis method.
In water treatment processes based on the reverse osmosis method, a pressure corresponding to the osmotic pressure induced by the dissolved solutes should be applied to the feed water in order to separate the solutes such as salts (e.g., NaCl) from the water. For example, the concentration of salts dissolved in seawater is 30,000 ppm to 45,000 ppm, and the osmotic pressure induced thereby is approximately 20 atm to 30 atm. As a result, in order to produce fresh water from the feed water, a pressure exceeding 20 atm to 30 atm has to be applied to the feed water. Thus, energy of at least 6 kW/m3 to 10 kW/m3 is generally required to produce 1 m3 of fresh water from seawater.
Although an energy recovery system has been developed recently and used to reduce energy consumption in the reverse osmosis process, even in that case, energy of at least about 3 kW/m3 is required to drive a motor of a high-pressure pump. To solve this problem, a water treatment method using a forward osmosis membrane has recently been presented as a solution.
The forward osmosis method is used for the membrane separation through the phenomenon in which the solution with the low concentration moves to the solution with the high concentration. Since the forward osmosis method utilizes the natural osmosis phenomenon, it is very economical as compared to the reverse osmosis method because it is not necessary to apply a separate pressure. Thus, the development of the forward osmosis membrane is being actively carried out recently. The forward osmosis method that is a contrary concept of the reverse osmosis method may be different from the reverse osmosis method in separation membrane. Thus, it may be difficult to apply the reverse osmosis membrane to the forward osmosis membrane.
In the forward osmosis method, water as a draw solution may be well introduced from feed water through the membrane, whereas a concentration of a draw solute has to be uniformly maintained, and simultaneously, a high osmotic pressure has to be maintained.
For this, it may be very important to design the forward osmosis membrane so that the forward osmosis membrane has to have high water permeability in an osmosis direction, and the solute of the draw solution is not diffused in a reverse osmosis direction. Also, the manufacture of the forward osmosis membrane having relatively low membrane contamination has to be preceded.
Essential conditions of the forward osmosis membrane are as follows.
First, in order to minimize an internal concentration polarization to improve fouling resistance, a support layer within the forward osmosis membrane has to have high porosity, and a pore has to have low tortuosity. Second, in order to increase a flux of water to be penetrated, a thickness of the forward osmosis membrane has to be minimized. Third, in order to minimize water penetration resistance, a material having hydrophilic property may be used. Fourth, in order to maintain the draw solution having the high concentration, the solute does not have to be diffused from the solution having the high concentration to the solution having the low concentration.
Even if the above-described forward osmosis membrane is developed and used, it may be difficult to achieve the object for the separation of the fresh water from the seawater through the maximum osmotic pressure gradient. To realize the forward osmosis method, the above-described forward osmosis membrane has to be essentially provided. In addition, it may be necessary that the seawater smoothly flows into the forward osmosis membrane so that the osmotic pressure gradient with respect to the draw solution flowing outside the forward osmosis membrane is well generated.
In the related art, a channel having one spacer is formed so that the seawater smoothly flows into the forward osmosis membrane. Particularly, FIG. 1 is a perspective view of a forward osmosis membrane assembly according to the related art. Referring to FIG. 1, one spacer 2 is provided between forward osmosis membranes 1 and 3. However, if the one spacer is provided as described above, the smooth introduction of the seawater into the forward osmosis membranes, which is capable of maximally generating the osmotic pressure gradient may be difficult, and thus, it may be difficult to achieve the desired flux.
Also, Korean Patent Application No. 2010-7023340 discloses a spiral wound membrane module for forward osmosis. Here, at least one spacer is simply provided, but limitations thereof are not defined. Also, the at least one spacer is provided to achieve a desired height, volume, and flux, or other variables in a fluid flow route, but their effects are not specifically described.
However, for the sole reason in which the spacer is provided in plurality within the forward osmosis membrane without defining its limitation as described in the foregoing specification, the flux is not necessarily increased through the desired superior osmotic pressure gradient. If the spacer is provided without considering the number of spaces and a thickness of each of the spacers, the spacer may have an influence on an area of the forward osmosis separation membrane, which is capable of generating the osmotic pressure gradient, of the limited forward osmosis module. If the spacer has a thickness exceeding a predetermined thickness, the forward osmosis separation membrane may decrease in area to cause deterioration of the osmotic pressure gradient.
Furthermore, a fluid may flow between the inside and outside of the forward osmosis separation membrane, which have to be fluidically isolated from each other, due to an adhesion defect of the forward osmosis separation membrane. Thus, a specific study on a spacer that is capable of solving the above-described problem and also achieving a maximum flux is urgent.